MISCELLANEOUS MEASUREMENT ACOUSTICS MEASUREMENT Sound waves or acoustic pulses are vibrations that consist of a succession of rapid variations in air pressure, usually of small magnitude. These are transmitted through the medium and produce a sensation of hearing in the hearing mechanism. Sound Waves are classified into following categories. 1. Infra sound: Sound waves having frequency less than 20 Hz is called as 'infra sound'. 2. Audible sound: Sound waves having frequency between 20 Hz to 20 kHz is said to be ‘audible sound'. 3. Ultrasound: Sound having frequency more than 20 kHz is said to be 'ultrasound'. Sound forms an important part in human environment. By means of sound we (can communicate, warn danger, enjoy music etc. The unpleasant sound is termed as noise. Excessive noise causes irritation and annoyance. Acoustic is a branch of [physics that deals with the design, generation, detection and propagation of sound waves. Acoustical measurements find wide application in our life e.g. noise control study, development of less noisy machinery and equipment, design and testing sound recording and producing equipment, study of aerodynamic noise. Characteristics of sound Intensity or Loudness The intensity (I) of a sound wave at a point is defined as 'the energy flowing per second per unit area held normally at a point to the direction of the propagation of sound wave'. It is expressed in W/m2 or J/m2. It is purely a measurable quantity. The intensity of sound wave is given as,

Where, Q is sound energy, t is time, A is area of sound source We know that,

Weber and Fechner Law: Loudness (L) is the degree of sensation produced in our ears. The loudness of a sound wave varies from one listener to another. It is a psychological quantity and, therefore, it is difficult to measure because it depends upon the individual listener. Loudness of a sound wave is directly related to intensity (I) and is proportional to Log I.

Greater the intensity of a sound wave, greater is its loudness.

The magnitude of the sound intensity is directly proportional to the logarithm of the sound intensity. This is known as 'Weber and Fechner law'. Measurement of Intensity of Sound Wave: The absolute intensity of a sound wave is measured in W/m2. For all practical purpose we are interested in relative intensity rather than absolute intensity. The standard intensity selected for this purpose is given by

This standard intensity is known as threshold of audibility for a note of frequency 1000 Hz. The intensity level of sound wave is a way of stating the ratio of its intensity relative to the standard intensity. The relative intensity or intensity level of a sound wave having intensity T is given by, The unit of intensity level (β) is 'bel'. In practice, 'bel' is a large unit. Hence, another unit known as decibel (dB) is more often used. 1 bel = 10 decibel 1 bel = 10 dB Normally, ear can detect the intensity level within the range of 0-120 dB. The loudness of 120 dB produces a feeling of pain in the ear and is therefore called as "threshold of feeling'. Frequency or Pitch Frequency is defined as 'the number of vibrations produced per second'. The greater is the frequency of a musical notes the higher is the pitch and- vice versa. The frequency is a measurable physical quantity and it can be measured accurately, while the pitch of note is a psychological quantity which is merely a mental sensation experienced by us. The pitch is directly proportional to the' frequency. Generally musicians describe the fixed frequency in terms of pitch of the musical note by a set of letters, e.g. the pitch of note produced by 256 Hz sound is represented by middle C. Pressure or Sound Pressure Level Sound pressure level (SPL) is defined as '20 times the logarithmic ratio in base 10 of the sound pressure of a sound to a reference pressure'. In mathematical form SPL is given by,

Where P is the sound pressure of a sound and P ref is the reference pressure. The unit of SPL is decibel. The sound pressure of 2X10-5 N/m-2 is the most commonly used reference level. The advantage of using logarithmic scale in defining the SPL is that it permits a large range of pressure to be described with convenience. SPL is measured using a sound level meter.

Power or Sound Power Level: Sound power is the total sound energy radiated by a sound source per unit time. The power in the sound emitted from a source can have an extremely large range. The power can be as low as 10~9 W at a faint whisper or 10-5 W at normal conversational speech and it could be as large as 10 MW for Saturn rocket or 50MW for jet airliner. Because of this, wide range of values of power in the various power sources, the sound power level is conveniently defined in the units of decibels. The sound power is often abbreviated as PWL or L w and is defined as,

Where, W is acoustic power of the source. W ref is the reference acoustic power (W ref = 10-12 W). Since the power ratio 10-12 means - 120 dB, the above equation is written as, Where W is in Watts. SOUND LEVEL METER Sound pressure levels are measured by sound level meters. Sound level meter convert acoustic pressure into a directly proportional voltage indicated on a meter. The meter is positioned at a desired location with no obstruction from the sound source and the reading is taken. The block diagram of instrument can be shown as, Sound level meter includes microphone, electronic amplifier with frequency weighting network and a meter or recorder calibrated, in decibels. A rectifier circuit is incorporated to produce a signal proportional to RMS value. The output of the meter is in the form of averaged root mean square (RMS) value of the signal. The frequency response of un-weighted system is generally flat or linear over entire available range from 20 Hz to 20 kHz. A frequency weighted network is used in the circuit to provide a response similar to that of the human ear. With the network, there is different amount of amplification for each frequency so that the overall measurement made gives greater emphasis to some frequency than to others. It is seen from loudness contours that the ear is more

responsive to frequencies between 500 to 5000 Hz than those above or below these frequencies. Three standard weighted networks A, B and C are used. Piezoelectric Crystal Type Meter Certain material possesses the ability to generate an electrical potential when subjected to mechanical strain. Such materials are generally crystalline in nature and are known as yPiezo electric crystal'. The commonly used materials are Quartz, Rochelle's salt (potassium-sodium tartarate), Lead zinconate titanate, Barium titanate and Ammonium dihydrogen phosphate. Rochhelle's salt has strongest piezoelectric effect and is widely used as microphone transducer. But it requires protection from moisture in air and cannot be used above 50°C. For the greatest sensitivity, a cantilever type of crystal is mechanically coupled with sensing diaphragm is used. Piezoelectric microphones are very sensitive and can measure accurately sound pressure level below 24 dB. Their response to lower frequencies is also good.

Humidity measurement Humidity is the amount of water vapour present in air. Humidity is important parameter for efficient and economic industrial process therefore it is required to be controlled. Moisture affects the behaviour of many commercial materials like paper, textile, paints, resins, soap powder, fertilizer, leather, film, wood, biscuits, sugar, salt etc. Humidity is important in following processes. 1. Air conditioning applications. 2. Food process industry. 3. Printing press. 4. Textile. 5. Chemical processing plant. 6. Fertilizer industry. Atmospheric air always contains water vapour. The amount of water vapour present plays an important role in comfort air-conditioning. The science, which deals with study of behaviour of air and water vapour mixture, is known as psychrometry. Various properties of air and water vapour are known as 'psychrometric properties of air". Various psychrometric properties like dry bulb temperature, wet bulb temperature, dew point temperature, specific humidity, relative humidity, enthalpy etc. are represented on graphical chart is known as 'Psychrometric chart'. If we know any two properties, we can locate point on psychrometric chart and rest of the properties can be found out very easily in fraction of second. Dry air: Dry air is the mixture of nitrogen (77%) and oxygen (23%) neglecting water vapours and other gases. Moist air: Moist air is mixture of dry air and water vapour. The amount of water vapour present in the air depends on the absolute pressure and temperature of mixture. Saturated air: The mixture of air and water vapour is said to be saturated when it contains maximum amount of water vapour that it can hold. 4 Dry bulb temperature: It is the temperature of air recorded by ordinary thermometer.

Wet bulb temperature: The temperature recorded by thermometer when its bulb is covered by wet cloth (Wick) is known as Wet bulb temperature. Dew point temperature: If mixture of air and water vapour is cooled at constant pressure then the ability of air to hold water will reduce and it will approach saturation condition. Further removal of heat results in condensation of water vapour or formation of dew. Thus, the temperature at which formation of dew starts is known as 'dew point temperature'. Humidity: Humidity is the moisture content in air. It may be expressed as specific humidity in kg/kg of dry air, absolute humidity in gm/m3 or relative humidity Sling Psychrometer: The equipment used to measure dry bulb and wet bulb temperature simultaneously is known as "Psychrometer1. The sling Psychrometer consists of two mercury thermometers mounted on frame, which has handle provided for rotation of Psychrometer. Thus, air movement is obtained by whirling the thermometer in air. One bulb among the two is covered with the wet wick to read wet bulb temperature. The velocity recommended for rotation is 5 to 8 m/min.

Hair Hygrometer: Hair hygrometer is cheap pocket size instrument used for humidity measurement. Certain materials such as human hair, animal membrane, wood and paper undergo changes in linear dimensions when they absorb moisture from the atmosphere. Human hair becomes longer as the humidity of the surrounding air increases, and shortens when the air becomes dry. This property of hair can be used to operate a pointer or recording pen through a system of mechanical linkage. The indicator scale can be calibrated to give direct indication of the humidity. In hair hygrometer, transducer element consist of strands of hair to give it increased mechanical strength. The hair strands are generally arranged parallel to each other with sufficient space between them for giving free access to the hair sample under test. For proper function the element is maintained under light tension by the spring. Hair hygrometer is not very precise instrument but can be used between temperature limits 5 to 35°C for humidity range 40 to 90%.

Humistor Hygrometer Humistor is a short form of Humidity sensitive resistor. The electrical resistance of such resistors is found to vary particularly with the changes in relative humidity of surrounding air and therefore these are used as sensing element of humidity measurement. As relative humidity of surrounding air changes, electrical resistance of humister also changes. The humister consists of two metal grids bounded to a sheet of plastic. This arrangement is coated with moisture sensitive salt such as lithium chloride. Other salts like barium fluoride, potassium hydrogen phosphate or aluminum oxide can also be used. As relative humidity increases, the film becomes more conductive and electrical resistance of the grid decreases. The variation in resistance is calibrated in terms of relative humidity units. It has been found that a single transducer generally cover small range of Relative humidity. In order to cover humidity range from 5 to 99%, a combination of seven or eight transducer elements is used, each designated for a specific part of total range. Since the humister is an electrical sensor, it meets the industrial need of speed, versatility, accuracy and high sensitivity. It can detect the 1% change in humidity. Advantages of Humistor: 1. It meets the industrial need .of speed. 2. Versatility and accuracy. 3. High sensitivity. 4. Capable of detecting a fraction of 1 % change in relative humidity. LIQUID LEVEL MEASUREMENT Liquid level measurement is widely used for monitoring as well as measuring the quantity of liquid content in vessel,' reservoir and tanks. The type of device used for Iiquid level measurement depends upon accuracy, repeatability, range. For industrial purpose wide range of liquids such as water, solvents, chemicals, lubricants etc. are used in many processes. The liquid level in container affects the pressure and flow rate. Therefore, its measurement and control is important modern industrial process. Apart from this liquid level measurement, it is very useful where human reach is not possible like: 1. Water level in boiler. 2. To detect level of molten metal. 3. To detect the level of harmful.chemicals in chemical industries. 4. To detect sewage level. 6.5.1 S i g h t G l a ss Sight glass tube is used for continuous indication or measurement of liquid level. It consists of a toughened glass tube mounted on one of the side of»tank such that it is parallel to the tank wall. The measuring scale is graduated on the glass tube. The liquid whose level is to be measured is poured into tank; the liquid level in sight glass also rises. When some liquid is taken out of container, the level in sight glass also decreases. Thus, level in container is directly indicated by level in sight glass. Measurement and Control 6-10 Miscellaneous Measurement Sight glass

Open tank liquid level L/ Scale iTTZ1M - High pressure Valve IT : h , Glass tube . ^Lwith engraved * ^ markings (a) Sight glass when used for (b) Sight glass when used for open container closed container Fig. 6.4 : Sight glass Sight glass when used for high pressure container : When it is required to measure level irt high pressure container, sight glass is connected to container such that one end of it will open in high pressure region (above liquid level) and another end of sight glass open in below liquid level. Two valves are provided to isolate the sight glass from container. When liquid level is required to measure the valve isolating sight glass are opened and reading is taken. To read the liquid level, scale is provided or markings are engraved on glass tube. Some times the spherical steel balls are used for blocking flow of, in case glass tube is broken. Advantages of Sight Glass : 1. It is simple in construction. 2. Its cost is low. 3. It is suitable for continuous indication. Disadvantages of Sight Glass : 1. Accuracy depends on cleanliness of liquid. 2. The liquid inside tube in cooled weather may freeze. 3. It is required to mount parallel to wall of container. 6.5.2 Float Gauges The float gauges works on buoyancy method to give direct continuous reading. The float rests on the surface of liquid and follows the change in level of liquid. This movement of float-is transferred to the pojnter through some suitable mechanism. The different types of floats can be used are hollow metal spheres, cylindricaUshape or disc shape floats. The mechanism used for transferring movement of float to pointer is shown in Fig. 6.5. The simplest mechanism consists of cable wound over pulley. One end of cable is provided with" float, floating on liquid level and another end of cable is provided with balance weight. With the change in level of liquid, float moves causing pulley to Measurement and Control 6-11 Miscellaneous Measurement move. The pointer is mounted on pulley which moves over calibrated scale as shown

i Fig. 6.5 (a) or pointer may be attached to cable which moves over scale as shown m Fig. 6.5 (b). Calibrated scale Pointer Pulley Cable Pulley Float rw—Balance weight Liquid level /—-\ Balance I *T weight (a) (b) Fig. 6.5 : Simple mechanism of float Advantages of Float Gauge : 1. Liquid level in tank can be obtained even if tank is kept below ground level. 2. It is reliable having low cost. 3. Suitable material float can be selected to avoid corrosion. Disadvantages of Float Gauge : 1. It is required to be design according to tank geometry. 6.5.3 Float and Shaft [S-09, S-r11] These are similar to float gauges, which utilizes float to measure liquid level. Instead of attaching float to cable it is attached to shaft. Another end of shaft is pivoted therefore with the change in liquid level float rises or lowers freely. This movement of float is communicated to pointer through gear and sector arrangement as shown in Fig. 6.6. Calibrated scale y /Pointer .Pinion i . {^J^ Liaui Sector Liauid level Fig. 6,6 : Float and Shaft gauge Measurement and Control 6-12 Miscellaneous Measurement These are similar to float gauges, which utilizes float to measure liquid level. Instead of attaching float to cable it is attached to shaft. Another end of shaft is pivoted therefore with the change in liquid level float rises or lowers freely. This movement of float is communicated to pointer through gear and sector arrangement as shown in Fig. 6.6. Advantages of Float and Shaft Gauge : 1. Mechanical advantage can be obtained.

2. It is excellent for precise control action. Disadvantage of Float and Shaft Gauge : 1. It can not be used for wide range of liquid level measurement. 6.5.4 Bubbler or Purge System The bubbler or Purge system works on the principle of calculating the hydrostatic pressure by allowing the bubbles of air to flow through the liquid and converting this hydrostatic pressure into level of liquid. Bubbler system consist of simple hollow tube immersed in tank in which measurement of liquid is to be done. Two connections are provided to bubbler tube, one to air supply reservoir and another to pressure gauge to measure hydrostatic pressure. Regulating valve is provided to control the supply of air. Pressure gauge | calibrated to read liquid level Bubbler tube Tank Fig. 6.7 : Bubbler or Purge system When tank is empty or no liquid is present below the bottom end of tube, air passed through the tube will show zero gauge pressure. When tank is filled with some liquid and air is passed through the tube, the liquid level in tank provides the •easurement and Control 6-13 Miscellaneous Measurement resistance to air and pressure gauge will show some pressure which can be calibrated in terms of liquid level. As the bubbles are continuously coming out of tube and liquid do not enter into tube, the tube is said to be purged. The common purging fluid is air but if use of air is objectionable different gases like nitrogen or carbon dioxide can be'used. Advantages of Bubbler or Air Purge System : 1. It is simple assembly. 2. It can be used for corrosive liquids. Limitations of Bubbler or Air Purge System : 1. It requires air lines to carry air. 2. Danger of bursting of air pipe. 3. It can not be used for measurement in pressurized vessel. .' 6.5.5 Float Operated Rheostat Rheostat Pointer Fig. 6.8 : Float operated rheostat Float operated rheostat method of liquid level measurement is as shown in ~\g. 6.8. It consists of float pivoted to wiper as shown in figure. With the change in iquid level float rises or lowers. This movement of float causes the wiper to move ind change the resistance of rheostat. Thus, circuit resistance changes which Ganges output of circuit. Thus, output of circuit is calibrated to give level of liquid. 6.5.6 Capacitive System [W-08, W-09, W-10] In this type of liquid level measurement method, metal tank is used in which the

liquid is filled whose level is to be measured. The insulated capacitance probe is firmly fixed near and parallel to wall of tank. If liquid in tank is non-inductive (insulator), the wall of tank and capacitance probe forms a parallel plate capacitor and liquid inbetween act as dielectric fluid. If liquid is conductive, the capacitance probe and liquid form a capacitor and insulation acts as dielectric. The metal tank and capacitance probe form two terminals of capacitance measuring instrument which is calibrated to read the liquid level as capacitance changes with change in level of liquid. When level of liquid in tank rises, capacitance increases and vice versa. Measurement and Control 6-14 Miscellaneous Measurement Capacitance measuring instrument calibrated in terms of liquid level Insulation Capacitance probe •—Metal tank Fig. 6.9 : Capacitive liquid level measurement type system The principle of operation of capacitive level measurement system is equation of parallel plate capacitor. K A C = ~D Where, C = Capacitance in farad, K = Dielectric constant, A = Area of plate in mm2 . D = Distance between two plates in mm. From above equation if 'A' and ' D ' are constant, then capacitance of capacitor is directly proportional to dielectric constant. Advantages of Capacitive Liquid Level Measurement: 1. It is very useful for small systems. 2. It is very sensitive. 3. It gives continuous indication. 4. It can be used with slurries. 5. It is simple to design and lower cost. 6. It requires less maintenance. Disadvantages of Capacitive Liquid Level Measurement: 1. Its sensitivity is affected by temperature. 2. Measured liquid should have dielectric qualities. 3. Performance is affected by dirt and other contamination which changes dielectric properties of liquid. asurement and Control 6-15 Miscellaneous Measurement 6.5.7 Gamma Ray Liquid Level Sensor [S-10]

It is contactless method of measuring liquid level. ler methods of measurement are not suftable. Gamma source holder/ It is generally used where Tank x band of N N VSamma ra/^s Liquid lefrel N —v- •To indicating instrument Operating range Detector Fig. 6.10 : Gamma ray liquid level sensor It consists of tank in which liquid is filled whose level is to be measured. A gamma ray source holder is placed at one of top side of tank and receiver is placed fcactly opposite bottom side of tank to receive the gamma rays emitted by source, •he gamma ray source holder is mounted such that it covers the entire wall of tank. •Then gamma ray source holder is switched 'ON' it transmits the thin band of gamma lays, which covers the entire tank wall and passes through medium in tank. A ladiation detector is placed parallel to the tank wall, which will detect a radiation Ifevel of gamma rays. It converts the radiation level into its equivalent electrical pgnal, which will be measured by measuring device. The reading shown by peasuring device is directly calibrated in terms of liquid level. When the gamma ray source is switched 'ON' and tank is empty, then gamma •ays passes through air in the tank and tank wall. As a result of this, its energy level decreases and detector detects the radiation energy level. When liquid level is to be peasured, gamma rays passes through liquid and tank wall, due to which radiation level will again decrease which is detected by detector. Therefore the difference Between the two readings shown by detector is directly proportional tb the rise of Squid level in tank. Advantages of Gamma Ray Liquid Level Sensor : • 1. "Its accuracy is very high. 2. There is no physical contact with liquid. Measurement and Control 6-16 Miscellaneous Measurement 3. It has high sensitivity. 4. It can be used for measurement of level of hazardous liquid. 5. Its design and construction is simple. Disadvantages of Gamma Ray Liquid Level Sensor : 1. It is very costly. 2. Radiation source holder may be heavy and bulky. 3. Mounting of gamma ray source holder may be difficult. 6.6 FORCE MEASUREMENT

In our daily life we see that bodies at rest are moved or bodies in motion are brought to rest. In order to move bodies the work is to be done. For doing this work, force is exerted on bodies to achieve the results. Hence, the force can be defined as 'an action that changes or tends to change the state of rest or motion of body, upon which it acts'. Force is specified by its magnitude i.e. quantity of force, direction, point of application i.e. line of action and sense i.e. push or pull. Unit of force : SI unit of force is Newton. MKS unit of force is kgf. Basic Methods of Force Measurement: The unknown force can be measured by following ways. 1. By balancing the unknown force against known gravitational force either by directly or indirectly. 2. By measuring the acceleration of body of known mass to which the force to be measured is applied. 3. Balancing unknown force against magnetic force developed by current carrying, coil and magnet. 4. By balancing the unknown force to fluid pressure. 5. By applying unknown force to an elastic member and measuring resulting deflection. 6. By measuring the change in natural frequency of wire which is applied with force. Tool Dynamometer (Mechanical Type) In metal cutting process it is important to know the magnitude and direction of all the forces involved in the process. To design the process, cutting tool and machine tool accurate measurement of cutting force is required. Reasons to Measure Cutting Forces: 1. To find power required for metal cutting. 2. To estimate forces coming on machine tool. 3. To measure tool wear and tool life. 4. To find relation between different components of force. 5. To design and control machining process. Tool dynamometer is one of the methods to determine cutting force. The forces .involved in traditional (Oblique) metal turning process are shown below. feed. Fr = Radial force or thrust. Radial force acts along the radius of work piece. It is the smallest force acting along the axis of tool. Fc * Cutting or tangential force. Cutting force acts in vertical plane and is tangential to the work surface. It is the largest force in cutting operation. Fc n - Cutting normal force The resultant force is given by, R = V(Ff)2 + (Fr)2+ (Fc)2

For Orthogonal cutting system, which has two dimensional force system. . R = V(Ff)2 + ( F c)2 Cutting force depends on : "1. Work piece material 2. Depth of cut 3. Feed • 4. Tool geometry 5. Cutting fluid Thus, we have two basic cutting processes (i) Orthogonal cutting and (ii) Oblique pitting. Measurement and Control 6-18 Miscellaneous Measurement tdeasurei The difference between these processes is : [W-09] Orthogonal cutting Oblique cutting 1. Cutting face of the tool is perpendicular to the direction of feed. 2. It is known as two-dimensional cutting. 3. Resultant-cutting force, 1. Cutting face of the tool is inclined at angle less than 90. 2. It is known as three-dimensional cutting. 3. Resultant cutting force, R = V(Ff)2 + (Fc)2 4. Tool life is less. R = V(Ff)2 + ( F r ) 2+(Fc ) 2 4. Tool life is more. To determine the various cutting forces in metal cutting, tool dynamometer is used. The different types of tool dynamometers are: 1. Mechanical type tool dynamometer 2. Strain gauge type tool dynamometer 3. Pneumatic and hydraulic tool dynamometer 4. Capacitive pick-up tool dynamometer 5. Electrical tool dynamometer The constructional features of typical tool dynamometer are based on principle that force can be measured through its action on system offering a finite resistance that is usually pre-calibrated. The measurement system involves: (a) Transducer.

(b) Signal conditioner to convert transducer output into measurable parameter. (c) Indicator or recorder to give final results. Requirement of Tool Dynamometer: 1. The dynamometer should be sufficiently rigid to avoid excessive deformation of the cutting edge under action of cutting force. 2. The dynamometer should have sufficient sensitivity to enable measurement with good accuracy. 3. The dynamometer should be of high stiffness and low mass, ensuring 100 % transmission of force by its natural frequency. Vibrations should not affect its performance. 4. Dynamometer should be capable of indicating individual force components: 5. The measuring system should be stable with reference to time, temperature and humidity. 6. It should have simple design and easy to use. 7. It should not provide any disturbance or obstruction to cutting process. MECHANICAL TYPE TOOL DYNAMOMETER The simplest type tool dynamometer is as shown in fig. It consists of sensitive dial indicators for directly measuring of forces. The dial indicator is calibrated to read the cutting force. The calibration is done by considering the deflection. The deflection produced is directly proportional to the cutting force. The dial indicator is the instrument used to measure or checking fine linear dimension It consists of a graduated circular scale, plunger and gear pinion. The dial indicators are placed in direction of cutting force to find deflection. SHAFT POWER MEASUREMENT Dynamometer is the device used to measure the shaft torque or power. Generally, shaft power is measured by measuring speed and torque of shaft. The power is given as P=2πNT. Or we can say that power is measured by measuring torque, by measuring force and radius. Different types of dynamometers are: 1. Absorption dynamometers: In this type, energy is converted into heat by creating friction. The produced heat is dissipated to surrounding and it goes waste. Absorption type dynamometers are used for measurement of power of engine, turbine and electric motor. The different types of absorption dynamometer are: (a) Mechanical type: Proney brake (b) Hydraulic type: Hydraulic absorption dynamometer (c) Electric type: Eddy current dynamometer 2. Transmission dynamometer: In transmission dynamometer the energy transmitted either to or from dynamometer is either absorbed or dissipated. After measurement, the energy is

transferred to the surroundings in a useful mechanical or electrical form. But small amount of power may be lost by friction in dynamometer. Transmission type dynamometer are : (a) Mechanical type: Torsion and belt dynamometer (b) Strain gauge dynamometer 3. Driving dynamometer: These are coupled to either absorbing or power generating devices and it may operate as motor or generator. This device measures the power and also supplies energy to operate tested devices. They are generally used to determine the performance characteristic of machine such as pump, compressor, e.g. Electric cradled dynamometer. EDDY CURRENT DYNAMOMETER Eddy current dynamometer is electrical absorption dynamometer working on principle that when isolated conductor moves through magnetic flux, it induces eddy current, which get dissipated in the form of heat. It consists of toothed non-magnetic solid metallic rotor connected to the shaft whose power is to be measured. The non-magnetic rotor rotates inside smooth cast iron stator. The stator is provided with exciting coil of D.C. source. The stator is mounted such that it permits free swing about its axis (Cradled) provided with torque arm, which measures torque. To dissipate the generated heat, water is supplied in stator casing. During operation of dynamometer, rotor turns and causes constant change in flux density at all points of stator, resulting formation of eddy current, which opposes the motion of rotor. This opposing resistance is measured by brake drum in the form of torque, from which shaft power can be calculated. Advantages of Eddy Current Dynamometer: 1. It can measure high power output at all speeds therefore it is used to test automobile and aircraft engines. 2. It is compact as compared to other dynamometer of same capacity. 3. The torque developed is smooth and continuous under all operating conditions. 4. The absorption power can be changed by changing D.C. current. Advantages of Eddy Current Dynamometer: 1. It cannot produce any torque at zero speed. 2. It produces small torque at low speed. Application: It can measure power upto 300 HP with maximum speed 6000 rpm.

STRAIN GAUGE TRANSMISSION DYNAMOMETER This is inline rotating torque sensor, which measures torque. It consists of metal shaft fitted with bonded strain gauge. Strain gauges are fitted with angle 45° to the shaft axis. In this arrangement, two strain gauges are subjected to tensile stress while other two compressive stresses. Strain gauge 1 and 3 must be diametrically opposite to strain gauge 2 and 4. The strain gauge senses compressive as well as tensile deformation due to torsion. These strain gauges are connected to Wheatstone circuit. The output of Wheatstone bridge is proportional to torsion and hence to applied torque on shaft. The bridge power and output of bridge is connected the sensor through slip ring and brushes. Slip rings are conducting rings but insulated from shaft. One slip ring is connected to each of bridge terminal. Advantages of Strain Gauge Dynamometer: 1. It has full temperature compensation. 2. It provides automatic compensation for bending and axial loads. 3. It is sensitive to torque. SPEED MEASUREMENT Measurement of speed is important in machine tool and textile machinery. The speed can be measured with tachometer in most convenient manner. For the shaft, which is not accessible to the tachometer, its speed can be measured by frequency type tachometer. Definition of Tachometer: The tachometer may be defined as: (a) An instrument used for measurement for angular velocity of shaft either by registering the total number of revolutions during the period of contact or by indicating directly the number of revolutions per minute. (b) An instrument, which either continuously indicates the value of rotary speed or continuously displays a reading of average speed over short intervals of time. MECHANICAL TACHOMETER Mechanical tachometer, employ only mechanical parts and mechanical movements for the measurement of speed. Types of Mechanical Tachometer: (a) Revolution counter and timer. (b) Tachoscope (c) Hand speed indicator. (d) Steeping clutch tachometer. (e) Centrifugal force tachometer.

Disadvantages of Mechanical Tachometers: 1. Mechanical tachometers impose load on the shafts to which they are connected. 2. On account of inertia of mechanical parts, the mechanical tachometers are not able to follow the variations in the speed. They indicate only the average speed over a period of time. 3. Mechanical tachometer normally uses an arrangement where a counter is used for counting the number of revolutions and stop-watch is used for measurement of time. The speed is calculated from the readings of two. This involves large error especially when stopwatch is operated manually. (a) Revolution Counter and Timer: Revolution counters are also, called as ‘speed counters'. It consists of worm and worm wheel. The shaft whose speed is to be measured is connected to worm. Along with shaft, worm rotates and depending on velocity ratio, worm wheel rotates. One rotation of worm causes the worm wheel to move by one tooth. A pointer on worm wheel can be directly calibrated to give number of revolutions made by shaft within prescribed time. From the total revolution in a given time, average speed is calculated. It is used to measure speed upto 3000 rpm. Advantages of Revolution Counter and Timer: 1. It is simple in construction. 2. It can be used conveniently to measure speed of shaft, pulley, gear etc. Disadvantages of Revolution Counter and Timer: 1. It gives average speed instead of instantaneous speed. 2. Its use is limited to slow speeds. (b) Tachoscope: In case of revolution counter and timer the difficulty is to start the revolution counter and timer simultaneously. The Tachoscope is the device which consists of revolution counter and timing device, when contact point is pressed against the rotating shaft it starts revolution- counter as well as timer. The average speed is calculated from the total revolution and time taken for revolution. It is used to measure speed upto 5000 rpm. (c) Hand Speed Indicator: Hand speed indicator has spindle, which can be attached to the- shaft whose speed is to be measured. The spindle is provided to with different types of spindle caps made of rubber to suit the various devices. The different types of points are conical, disc type or hollow cone etc. The spindle is connected to the mechanical counters, which display the count. The hand speed indicator has in-built stopwatch and automatic disconnect facility. Spindle operates when brought in contact with shaft but counter will function only when start or reset button is pressed. After a fixed time interval the revolution counter automatically gets disconnected. The instrument shows the speed over short interval of time and gets disconnected. Hand speed indicator has accuracy of ±1%. Generally, it is used for high speed measurement within range 20,000 to 30,000 rpm.

(d) Slipping Clutch Tachometer: In slipping clutch tachometer, clutch is used to connect the driving shaft with indicating shaft. Pointer attached to indicator shaft moves over calibrated scale. The rotating shaft drives an indicating shaft through a slipping clutch. A pointer attached to the indicator shaft moves over a calibrated scale against the torque of the spring. The pointer position gives a measure of the shaft speed. (e) Centrifugal Force Tachometer: The centrifugal force tachometer utilizes the principle of centrifugal force. The shaft whose speed is to be measured is connected to tachometer shaft. The two masses are attached to the pivoted arm, which rotate with tachometer shaft. The centrifugal force produced in mass tends to move it in outward direction. This movement is restricted by the masses attached to the shaft. The equilibrium position of the attached mass depends on the amount of centrifugal force produced in it. This upward movement of mass is communicated to pointer, which moves over calibrated scale. It is manufactured in various ranges and can be used to measure speed maximum upto 40,000 rpm. Advantages of Centrifugal Force Tachometer: 1. It is used to measure speed over wide range. 2. It has accuracy ± 1%. 3. It indicates the speed is constant of not which is not shown by hand speed indicator. Disadvantages of Centrifugal Force Tachometer: 1. Very high speed may damage the instrument. 2. It is to be placed in vertical direction. ELECTRICAL TACHOMETER Electrical tachometer converts the rotational speed of shaft into proportional electrical signal, which can be measured and calibrated to give rotational speed. The various electrical tachometers are: (i) Drag Cup Tachometer. (ii) Cumulated Capacitor Tachometer. (iii) Tachogenerator (i) Drag Cup Tachometer: Drag cup tachometer is electrical type tachometer, which works on eddy current. The shaft whose speed is to be measured is connected to permanent magnet at its end. A non-magnetic cup generally made of aluminum is provided very close to magnet, which is connected to pointer through spring. Rotation of magnet induces voltage in a cup and set circulating eddy current in cup material. This eddy current interacts with the magnetic field to produce a torque

on the cup in proportion to the relative velocity of magnet and cup. This causes the cup to turn through small angle. Low torque measuring transducer is used to measure torque and it is calibrated to get the speed of shaft. The various tachometers working on this principle are: 1. Mechanically coupled magnet type tachometer: This type of arrangement is as shown in Fig. The magnet is directly coupled to shaft whose speed is to be measured. The automobile speedometer works on this principle. 2. Stationary magnet: In this type of arrangement the permanent magnet is kept stationary and soft iron rotor is connected to shaft whose speed is to be measured. The soft iron rotor produces a revolving magnetic field. It is rugged in construction and generally used to measure locomotive speed. Advantages of Drag Cup Tachogenerator: 1. Linear characteristics of output voltage and speed of shaft. 2. It is rugged in construction. 3. It is inexpensive. 4. Requires very little maintenance Disadvantages of Drag Cup Tachogenerator: 1. To calibrate it input voltage must be maintained absolutely constant. 2. At very high speed, it gives non-linear characteristic of output voltage and speed of shaft. (ii)Cumulated Capacitor Tachometer: Cumulated capacitor type tachometer works on principle of alternately charging and discharging the capacitor. It consists of tachometer head having reversing switch, indicating unit, capacitor milliameter, calibrating circuit and D.C. supply. (iii) TACHOGENERATOR: Tachogenerator is an electrical type tachometer that generates AC or DC voltage O/P proportional to speed of shaft. It can be calibrated to give speed in terms of rpm, km/hr, etc.

It works on the principle of, ‘whenever rotating coil or conductor is placed in magnetic field, emf will induce in it, the generated emf is proportional to product of flux and speed’. Since the flux of permanent constant, the voltage generated is proportional to speed. There are two types of tachometer generators:

a) DC tachogenerator b) AC tachogenerator

(a) D.C. Tachogenerator: DC tachogenerator consists of armature, which is coupled to shaft whose speed is to be measured. The armature revolves in the field of permanent magnet. The emf generated is proportional product of speed and flux. As flux of permanent magnet is constant, emf is directly proportional to speed. The generated emf is measured by moving coil voltmeter which can be calibrated to give speed of shaft. The polarity of output voltage indicates the direction of rotation of shaft. Advantages of DC Tachogenerator: 1. Direction of rotation of shaft is indicated by polarity of output voltage 2. Output voltage is about 10mV/rpm and can be measured by conventional DC voltmeter Disadvantages of D.C. Tachogenerator: 1. The commutator and brushes requires periodic maintenance and their contact resistance may vary to produce error. 2. The input resistance of meter should be very high as compared with output resistance of generator. This is required to limit armature current to small value. If armature current is high, the field of permanent magnet is destroyed which results in non-linear relationship between voltage and speed. (b) A.C. Tachogenerator: In order to overcome some of the difficulties mentioned in D.C. tachometer generator an A.C. tachogenerator is used. A.C. tachogenerator has rotating magnet, which may be permanent magnet or an electromagnet. The permanent magnet is connected to the shaft whose speed is to be measured to form rotor of circuit. The stator of A.C. tachogenerator is cylindrical in structure having multi-pole piece, (generally four). The coil is wound on the stator and therefore the problem associated with commutator is absent. The rotation of the magnet causes an emf to be induced in stator coil. The amplitude and frequency of emf is directly proportional to the speed of rotation. Thus, either amplitude or frequency of this emf can be used to give rotational speed. The output voltage of A.C. tachogenerator is rectified and is measured by permanent magnet moving coil instrument.

Advantages of A.C. Tachogenerator: 1. No contact problems like D.C. tachogenerator. 2. Very less maintenance is required. Disadvantages of A.C. Tachogenerator: 1. Difficult to operate system at low speeds. The frequency of output voltage is low and hence it is very difficult to smooth out the ripples in output voltage wave shape, therefore A.C. tachogenerator is designed to have large number of poles so that the frequency of output voltage will be high even at lower speeds to increase accuracy. 2. High speed also has a problem. At high speed, the frequency increases and therefore the impedance of the coil of tachometer generator increases. If good linearity is to be maintained, the input impedance of the display device must be considerably larger than the impedance of the coil. CONTACTLESS ELECTRICAL TACHOMETER Contactless electrical tachometer does not have contact with shaft whose speed is to be measured. This tachometer produces pulse from rotating shaft, which is calibrated to give speed of shaft. The different contactless electrical tachometers are: (a) Inductive pick-up (b) Capacitive pick-up (c) Photoelectric Tachometer (d) Stroboscope (a) Inductive Pick-up: Inductive pick-up consists of permanent magnet with a coil wound on it. This permanent magnet is placed near the metallic toothed rotor mounted on shaft whose speed is to be measured. As rotor rotates; the reluctance of air gap between pick-up and toothed rotor changes resulting to an induced emf in the pick-up coil. This output is in form of pulse. The frequency of induced emf will depend upon number of teeth of rotor and speed of rotor. The number of teeth of rotor is constant; hence output emf is directly proportional to speed of rotor. Let, T is number of teeth on rotor, P is number of pulse per sec, n is speed of rotation in r.p.s. Then,

The typical rotor has 60 teeth. Thus, if the counter counts the pulse in seconds, the counter will directly display the speed in rpm. It is used to measure very low speed as well as very high speed about 70,000 rpm with very high accuracy. Advantages of Inductive Pick-up: 1. It is simple in construction. 2. It requires very less maintenance. 3. It is easy to calibrate. 4. It has very high accuracy. 5. It can be used for very low as well as very high speed measurement. (b) Capacitive Pick-up: Capacitive pick-up is used to measure rotational speed and translational motion of shaft. It utilizes the principle, 'Due to rotary motion, capacitance of variable capacitor is changed'. The resulting change in capacitance can be converted into useful electrical signal by means of various circuits. Capacitance type pick-up requires complicated electronics than that of simple transducers. The most common form of variable capacitor used is parallel plate capacitor with the variable air gap. For measurement of rotational speed of shaft, the shaft is attached with vane. When shaft rotates it changes the capacitance of the parallel plate condenser. Fig shows the basic form of a capacitive transducer utilizing effect of change of capacitance with change in distance between the two plates. One plate of capacitor is kept fixed and other plate is moved. The capacitance varies inversely with the distance’d’ between the plates of capacitor. The capacitance of such arrangement is given as,

C is capacitance, A is area of plate, d is distance between plate,

is permeability constant. Advantages of Capacitive Pick-up: 1. It has very small loading effect. 2. It has high sensitivity, Advantages of Capacitive Pick-up: 1. It has complicated electronic circuit. 2. It is used to measure only the small displacements. c) Photoelectric Tachometer:

The working principle of photoelectric tachometer is as shown in Fig. 6.24. An opaque disc having equidistant holes on its periphery is mounted on rotating shaft. At one Side of disc, a light source is fixed and on other side of disc in line of light source, light sensor such as pnototube is mounted. The opaque disc rotates along with shaft and when opaque portion of the disc is between the light source and sensor, the sensor produces an output pulse. The frequency at which these pulses are produced depends upon number of holes in disc and its speed of rotation. Since numbers of holes in disc are fixed, the output pulse is directly proportional to speed of disc and hence speed of shaft whose speed is to be measured. The output pulse is calibrated in, to give speed of shaft in rpm. It can be used to measure speed upto 10000rpm. Advantages of Photoelectric Tachometer: 1. The output signal is digital. 2. The electronic circuit is simple as pulse amplitudes are constant. 3. Can be used to measure very high speed accurately. 4. It is contactless method. Advantages of photoelectric tachometer: 1. A light source must be replaced time to time. 2. Suitable for very high speed measurement. i) Stroboscope: The stroboscope is simple, manually operated portable device which is used for measurement of speed. Stroboscope has variable frequency flashing light. An oscillator is provided to control flashing frequency. The speed is measured by adjusting frequency so that the moving object (target) is visible at specific intervals. If strong light is caused to flash on moving object, the object will appear stationary. The source of light is known as strobotron. Stroboton is hot cathode gaseous discharge tube. It consists of one cathode, one anode and two grids i.e. inner grid and outer grid. Conduction starts when potential, of outer grid is increased or potential of inner grid is decreased. The started conduction can be stopped by removing anode potential. The tube has capacity to flash 300 flashes per second. The flashing light is directed on rotating member, which usually has some spoke, gear teeth or some other feature. If rotating member do not have any of such features, a paper having black and white stripes is attached to it or some marking is done as a target. The frequency of lamp flashing is adjusted until the target appears stationary. Under this condition, speed is equal to flashing frequency. The scale of stroboscope can be calibrated to read the speed directly. If there are several marks on shaft, various errors may occur in measurement.

If disc has m number of marks, then disc will appear stationary,

The speed

Where, f = Number of flashes per sec. m = Number of marks on disc. To avoid confusion, approximate speed is calculated by other available methods. The value of exact speed can be found out as: Single line image is obtained by flashes. The flashing rate is gradually reduced and flashing frequencies are noted for all single line images. If single line images are obtained at m different flashing rates say f1, f2, f3, and f4 ….fm

fx = Lowest flashing frequency fm = Highest flashing frequency m = Number of flashing points or frequencies. The range of lamp frequency is 110 to 25,000 rpm. The accuracy of this method is ± 1 %. It is generally adopted for measurement of speed between 600 to 20,000 rpm. Advantages of Stroboscope: 1. It is contactless method. 2. It does not impose any load on shaft whose speed is to be measured. 3. It is useful method in which physical contact method cannot be used. 4. It requires no special attachment with shaft, Advantages of Stroboscope: 1. Variable frequency of oscillator cannot be stabilized to give fixed frequency. 2. This method is less accurate therefore requires the use of digital meters. 3. Stroboscope cannot be used where surrounding light is above some level. TYPES OF STRAIN GAUGES The strain gauges are used for: 1. To analyze dynamic strain in complex structure such as bridges, automobiles, roads etc. 2. To measure tension, torque, force and stresses in structures. 3. Measurement of force in load cell. Strain gauges are +ve passive transducer. If strip of elastic material is subjected to tension or positively strained, its longitudinal dimension will increase while there will be reduction in lateral dimension. Thus, the length increases and area decreases. As resistance is proportional to length and inversely proportional to area; the resistance of gauge increases with positive strain. It is +ve passive transducer as increase in the resistance of strain gauge wire is directly proportional to the mechanical stress applied on it. Classification of Strain Gauges Strain gauges may be classified depending upon the principle of operation and its constructional features as follows.

1. Mechanical gauges: These measures change in length and magnify it by using various mechanical elements like levers and gears. Mechanical strain gauges are comparatively larger in size. These gauges are used for static strain measurement. 2. Optical gauges: In these gauges magnification is obtained by using mirrors or prisms. The accuracy of these gauges is very high. In its simple form it consists of plain mirror rigidly attached to movable knife edge space. When subjected to stress, mirror rotates through an angle and reflect a beam of light. This reflection can be calibrated to give strain. 3. Electrical strain gauges: The principle of electrical strain gauge is based on measurement of changes in resistance, capacitance or inductance that is proportional to strain applied on test specimen. In general, resistance type strain gauges are used. It works on principle of change in resistance of wire with change in strain. The change in resistance is measured by using Wheatstone bridge. The output of Wheatstone bridge can be calibrated to give strain. GAUGE FACTOR: It is the ratio of change in resistance of strain gauge wire to its original value to per unit change in length.

i.e. Gauge factor is independent of applied strain. Requirement of Strain Gauge: While designing strain gauge to give accurate results of measurement the requirement of strain gauge is as follows. 1. Strain gauge should be small in size with negligible mass. 2. It should be highly sensitive to strain. 3. Strain gauge should have high value of gauge factor. 4. It should be easily attachable to specimen. 5. It should have high speed of response with negligible time lag. 6. It should be capable to indicate static, transient and dynamic strain. 7. It should be capable of remote indication and recording. 8. It should not be sensitive to ambient condition such as temperature, humidity, vibration etc. 9. It should be inexpensive, reliable and easily available in various sizes. STRAIN GAUGE MATERIALS: For fabrication of strain gauge, we require grid material, supporting or backing material and bonding material. Grid Material: Desirable characteristic of grid material: 1. It should have high specific resistance. 2. It should have high gauge factor. 3. It should have high electrical stability. 4. It should have low hysteresis. 5. It should have good solderability and weldability. The commonly used materials for grid are:

(a) Advance: It is 55 % copper, 45 % nickel having gauge factor 2. It is most commonly used as it has reasonable gauge factor. It can be easily worked and soldered. (b) Isoelastic: It is 36 % nickel, 8 % copper, 4 % Mn, Si and molybdenum and rest of iron. It has gauge factor 3.5. It has high gauge factor. It is useful in dynamic measurement. (c) Nichrome: It is nickel, chromium alloy having gauge factor 2. (d) Manganin: It has 0.47 gauge factor and low temperature coefficient. (e) Monel: It has high temperature coefficient and gauge factor as 1.9. (f) Nickel: It has negative gauge factor (-12). It exhibits reduced resistance though length increases and diameter decreases. Supporting or Backing Material: Desirable characteristics of support material: 1. Minimum thickness. 2. High mechanical strength. 3. High dielectric strength. 4. Minimum temperature restriction. 5. It should be non-hygroscopic. Generally thin strong paper is used for supporting or backing, Binding material: Desirable characteristics of binding material: 1. It should have high strength. 2. It should have high creep resistance. 3. It should have high dielectric strength. 4. It should have minimum temperature restriction. 5. It should have good adherence. 6. It should have ease in application. 7. It should be fast drying. Usually Duco cement is used as binding material. RESISTANCE STRAIN GAUGES: BONDED AND UNBONDED Depending upon its construction, strain gauges are broadly classified as : 1. Unbonded strain gauges. 2. Bonded strain gauges. (a) Wire type bonded strain gauge. (b) Foil type bonded strain gauge. (c) Semiconductor type strain gauge. 1. Unbonded strain gauges

Fig. shows the schematic representation of unbonded strain gauge, which consists of a fixed frame and movable frame or armature. Movable frame is connected to shaft which is able to move on either of the direction under the application of force or strain to be measured. Four strain gauge wires-having uniform cross-sectional area and equal length are connected in such a manner that one end of each wire is connected to the fixed frame and another end is connected to armature or movable frame. The resistance of wires Rs1, Rs2, Rs3, Rs4 are equal in magnitude. When unbonded strain gauge is used for measurement of force, stress or displacement then four strain gauge wires connected form a Wheatstone bridge circuit and detector. 'D' is calibrated in terms of the quantity being measured. The force, which is to measure, is zero then resistance offered by each strain gauge wire is equal in magnitude and detector shows the null position. When force to be measured is applied on shaft of gauge, due to applied force armature is moved. The movement of armature changes the resistance of strain gauge wire. Change in resistance of wire is detected by detector, which is directly proportional to quantity being measured. Advantages of Unbonded Strain Gauges: 1. It is simple in construction. 2. Its cost is low. 3. It is accurate if compared with cost. Disadvantages of Unbonded Strain Gauges: 1. Effect of hysteresis may effect on its performance. 2. Change in temperature affects its performance. 3. Resistance of contact lead decreases its accuracy. Application: It is used for measurement of force, strain or displacement. Bonded Strain Gauges: a) Wire type bonded strain gauge: Wire type bonded strain gauge consists of thin seat of insulating material such as paper or bakelite. A strain gauge wire having uniform cross sectional area and diameter about 0.025 mm is uniformly cemented on the seat of insulating material. The spreading of w i re permits a uniform distribution of stress. Two terminals taken out are called as connecting leads or terminal leads. Another thin seat of insulating material is placed on the strain gauge wire to prevent it from mechanical damage. The strain gauge is bonded with an adhesive material to the structure under study. This prevents a good transfer of strain from structure to strain gauge wire. When such type of strain gauge is used for measurement, it forms one arm of Wheatstone bridge network. When quantity being measured is zero, the resistance other arm is adjusted

such that bridge shows null position. When strain is applied, the resistances of s t r a in gauge wire changes causing unbalance condition in bridge. The deflection shown by detector is calibrated in terms of quantity being measured. This type of strain gauge is used as load cell. (b) Foil type bonded strain gauge: The construction of foil type strain gauge is exactly similar to that of wire type strain gauge but t h e se gauges have, more surface area as compared to wire type strain gauge. This causes increased heat dissipation capacity so t h a t it can be used at higher temperature areas. Generally, it is made in grid pattern. The grid element in the foil gauge is wider than its cross-section. The metals and alloys used for foil and wire are Nichrome, Constantan (Ni+ Cu), Isoelastic (Ni + C r + Mo), Nickel and Platinum. Foil type gauges can be fabricated in various sections. There is no stress concentration at the terminals due to absence of joints. Advantages of Foil Type Bonded Strain Gauge: 1. It has better thermal stability. 2. No stress concentration at terminals. 3. Can be fabricated in any size. (c) Semiconductor type strain gauge: Semiconductor strain gauge uses the piezo-resistive property (i.e. change in value of resistance due to change in resistivity) of doped silicon and germanium. The resistance of semiconductor changes with the change in applied strain. A typical strain gauge consists of a strain material and leads that are placed in protective box. It is bonded on suitable insulating material such as Teflon. Semiconductor type strain gauges have negative temperature coefficient of resistance due to which per unit change in length is very large compared to wire and foil type gauges. Sensitivity of semiconductor gauge is very high therefore, such type of gauges are used where very high accuracy is desired. It is generally used as one arm of Wheatstone bridge. Advantages of Semiconductor Strain Gauge: 1. It has high gauge factor therefore it allows measurement of very small strain. 2. It has excellent hysteresis characteristic, 3. Its fatigue life is more. 4. It is very small in length from 0.7 to 7 mm.

Disadvantages of Semiconductor Strain Gauge : 1. It is very sensitive to change in temperature. 2. It is more expensive and difficult to attach to the object under study. 3. It has non-linear characteristic. SELECTION AND INSTALLATION OF STRAIN GAUGES The proper functioning of strain gauge depends on quality of bonding, which holds the gauge to the surface of test specimen. The selection of strain gauge is done by considering its size and location where the strain gauge is to be mounted. The strain gauge is bonded with the cement on test specimen. The surface of test specimen should be smooth enough but not highly polished, as highly polished surface do not provide good adhesion. All scales, rust, paint and other surface contaminations must be removed. For surface preparation fine grid abrasive paper or emery cloth should be used for cleaning. The surface is further cleaned to remove any traces of grease or dirt using solvents like acetone, trichloroethylene with cotton. The strain gauge backing should be clean with solvent to remove oil and grease contamination and dried under electric bulb to remove moisture. A fairly generous layer of recommended cement is spread on prepared clean surface of specimen as well as on gauge backing. The gauge is then, lay at desire location very carefully, and pressed with thumb to exert pressure. Generally, pressure applied is 35 to 180 kPa and temperature 60°C to 200°C. A care must be taken to avoid air gap between gauge and specimen. LOAD CELL The term load cell is used to describe a variety' of force transducers which may utilize the deflection or strain of elastic member, or the increase in pressure of enclosed fluids. The resulting fluid pressure is transmitted to some form of pressure sensing device such as manometer or a bourdon tube pressure gauge. The gauge reading is identified and calibrated in units of force. Different types of mechanical load cells are: 1. Hydraulic load cell. 2. Pneumatic load cell. 3. Strain gauge load cell. Strain Gauge Load Cell: Load cell is application of wire type bounded strain gauge. It works on the principle of elasticity i.e. when axial force is applied, its column gets compressed and when force is released it regain its original position. Four wire type bonded strain gauges are cemented on column. Two of them are in horizontal position and two are in vertical position as shown in Fig. Gauges along x - x' are cemented in horizontal position whereas along y - y' in vertical position. The resistance offered by each gauge is same in magnitude. When it is used for measurement of axial force or strain, gauges a r e connected to form Wheatstone bridge network. When axial force applied is zero then the resistance of each gauge is equal in magnitude, which keep bridge in balance condition and deflection shown by detector is zero. When the axial force to be measured is applied on load cell then its column gets compressed. The compression of column causes decrease in resistance of strain gauge along y - y' and remains unaffected along

x – x’. This turns the bridge to unbalance condition. The deflection shown by detector can be directly calibrated to read axial force. Selection and capacity of load cell depends on: 1. Length and diameter of grid wire. 2. Thermal conductivity of grid wire. 3. Gauge factor of wire. 4. Strain of wire material. 5. Physical properties of wire material. 6. Cost of wire material. Advantages of Strain Gauge Load Cell: 1. Load cell is small and compact in size. 2. Accuracy is very good. 3. It has good sensitivity and low hysteresis. 4. It is less expensive. Disadvantages of Strain Gauge Load Cell: 1. Its performance is affected by non-axial force. 2. It requires temperature compensation network. 3. Excessive stress or force may damage load cell permanently. Application of Strain Gauge Load Cell: The strain gauge load cell is excellent force measuring device, particularly when force is not steady. It is used in industrial applications such as draw bar and tool force dynamometer, crane load monitoring, road vehicle weighing devices. ROSETTES For strain measurement in complex parts instead of single element strain gauge, combination of strain gauges are used, which is known as Rosette. Rosette is available in many combinations that can be used for strain measurement or as a transducer. These gauges have three or four separate grids with various angular orientations and they can be cemented to the part with no particular attention being paid to the overall gauge orientation. The resultant strain on each of the grid is recorded and true magnitude and direction of strain is calculated from collected data. Different types of two and three element rosette are shown below.

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